CO-tolerant RuNi/TiO2 catalyst for the storage and purification of crude hydrogen

Hydrogen storage by means of catalytic hydrogenation of suitable organic substrates helps to elevate the volumetric density of hydrogen energy. In this regard, utilizing cheaper industrial crude hydrogen to fulfill the goal of hydrogen storage would show economic attraction. However, because CO impurities in crude hydrogen can easily deactivate metal active sites even in trace amounts such a process has not yet been realized. Here, we develop a robust RuNi/TiO2 catalyst that enables the efficient hydrogenation of toluene to methyl-cyclohexane under simulated crude hydrogen feeds with 1000–5000 ppm CO impurity at around 180 °C under atmospheric pressure. We show that the co-localization of Ru and Ni species during reduction facilitated the formation of tightly coupled metallic Ru-Ni clusters. During the catalytic hydrogenation process, due to the distinct bonding properties, Ru and Ni served as the active sites for CO methanation and toluene hydrogenation respectively. Our work provides fresh insight into the effective utilization and purification of crude hydrogen for the future hydrogen economy.

The paper by Wang et al. tackles a combination of very interesting and hot subjects, namely COtolerant Catalyst development for Crude Hydrogen Storage via the LOHC technology. This reviewer ranks the experimental catalysis, microscopic and spectroscopic work in this paper as groundbreaking, however, the conclusion drawn from the CO-tolerant hydrogenation studies as well as its reaction mechanism need further improvement. This is why, this reviewer ranks the paper as 'major revision' but would strongly support publication in Nature communications after addressing the open points in the results analysis section in an appropriate manner: Firstly, there are some confusions: 1. there is some confusion with the "Mild Conditions" definition in the title of this manuscript. Toluene hydrogenation can be easily hydrogenated with pure H2 at reaction temperature below 100 oC. (ACS Omega 2021, 6, 8, 5846-5855: Under 30 °C, atmospheric H2 pressure, and a toluene/(Pt+Rh) molar ratio of 200/1, Pt0.77Rh1 alloy catalyst reach 100.0% of methyl cyclohexane). The definition of "mild conditions" should be consistent and correct throughout this paper with previous studies.
2. the statement in lines 57-59 is confusion too (However, as the benzene ring acts as the main unsaturated hydrogen storage unit, the essential ensemble-requirement of benzene hydrogenation would inevitably inhibit the application of single-atom catalyst in . The authors cite references here that deal mostly with metal particles catalyst (for which the statement is more correct). Single atom has been demonstrated to hydrogenate toluene with a higher reaction rate than particle catalyst. (Nat Commun 13, 1092(2022. https://doi.org/10.1038/s41467-022-28607-y;) Single atom catalyst has not been tested for toluene hydrogenation in the CO-tolerant case, but it has been claimed that a single atom (Pt) catalyst having a lower binding energy with CO than pt cluster/nanoparticles. Secondly, about the reaction pathway and mechanism. The authors provided a beautiful study on microscopic and spectroscopic side and experimental catalysis side. But only a FT-IR is not efficient to bridging them up for reaction pathway and mechanism discussion. 1. The author should show the results of selectivity. Is there any by products such as methyl cyclohexene or -diene? The author should make a discussion on that. 2. In heterogeneous catalysis, directly bridge reactant adsorption (FT-IR in this case) with reaction activity is not efficient. In some cases, suitable adsorption strength, as well as a pre-activation is important. Some stronger adsorptions may cause the poison effect. The authors should make a clear discussion on the specific case of toluene hydrogenation (in CO). (This study is a great case to talk about the adsorption-activation-reaction process and competing adsorption. and a clear discussion will strength this study to the level of Nature Communications.) 3. To further clarify the adsorption issue, this reviewer suggests that a toluene and CO Temperatureprogrammed desorption on the three catalysts sample should be provide and discussed. 4. The authors provided a lot of characterization data on catalytic materials side. This reviewer suggests the authors to combine these data with catalysis to propose a reaction mechanism model. Such as combine the metal-support interaction, and metal (Ru)-metal (Ni) interactions to further explain their distinct binding properties. And provide more insight for the conclusion of "Ru and Ni served as the active sites for CO methanation and toluene hydrogenation respectively." 5. Is the interface of Ru-Ni matter for the reaction?
Reviewer #3 (Remarks to the Author): The RuNi cluster on TiO2 wad design for stable activity for hydrogenation even with CO impurity. This paper can be accepted after considering followings.
1) Ru and Ni can form solid-solution alloy or separated cluster?
2) The electronic state of Ru and Ni can be modified by the alloying or clustering?
3)Why TiO2 is suitable support for the formation of this alloy cluster.
Reviewer #4 (Remarks to the Author): The manuscript describes the development of bimetallic RuNi@TiO2 catalysts with the anti-COpositioning ability for potential application in a LOHC-based hydrogen storage system, in detail the toluene-MCH pair and the concept was demonstrated in the hydrogenation step of toluene hydrogenation to MCH for H2 storage. This is an important topic for investigation and the findings from the current work do show the progress beyond the state of the art and provided some insights into the relevant mechanisms of the system under investigation. I am not the expert in all the areas included in the current work, but after reading it carefully, I identified the following comments/queries for the authors to consider for revision and improvement.
1. The title claimed mild conditions, and I think this should be reflected by a revised abstract and conclusion, i.e. < 200C at atmospheric pressure, indeed being much lower than the common conditions of 200-300 C and 10-50 bar. 2. Following the comment above, I think the authors need to explain why such high activity was achieved by the current catalytic system at quite short residence time since hydrogenation reactions normally requires elevated P and T. 3. The current catalytic systems using a bubbler for vapour phase reaction, being rather dilute. Can the authors try the conventional packed/trickle bed reactors or batch reactor with gas-liquid systems to show that the developed catalyst is not specific for the microreactor with gas phase reactions? 4. Exemplar GC analysis of products from the rig is needed in the revised SI. 5. Deactivation regarding MCH yield was measured during the longevity test, which was not discussed, though comment stating it stayed over 60% was provided. The authors need to elucidate this aspect since maintaining MCH yield is the key target of the work, yet continuous decline of the yield is very obvious. 6. Regarding the discussion for CO2 methanation, I think the authors oversell the performance. CO2 hydrogenation requires much higher temperatures than the current 100-220C, therefore, a simple discussion will be fine rather than being surprising about the phenomenon of non CO2 conversion in the current system. 7. Generic feature of the develop catalyst is needed, i.e. for hydrogenation of other LOHC pairs such as DBT to PDBT and/or benzene to cyclohexane.

Response to reviewers
We thank the reviewers for the very positive views and constructive comments. We have addressed all the comments point-by-point and revised the manuscript accordingly. In this response letter, comments from the reviewers are summarized with the major concerns quoted in blue italic.

Reviewer #1
This paper reports an interesting investigation of the simultaneous hydrogenation of CO (a poison) and toluene (a H-storage molecule). The topic is relevant to H-storage in liquid organic hydrogen carrier (LOHC). The paper is globally well structured and the results reported support the proposed conclusions that Ni-Ru/TiO2 are promising catalysts. There are yet two important aspects to consider in more detail.

Response to comments:
We thank the reviewer for the very positive view of our work. Per the request of the reviewer, we have added additional discussion in the following point-by-point response and the revised manuscript.

Response to comments:
We appreciate the reviewer's comment. As we know, the Ni(CO)4 will cause the Ni leaching from the catalyst. To clarify whether Ni(CO)4 is formed during the reaction, we compared the Ni contents of 2Ru5Ni/TiO2 catalysts before and after reaction by ICP-AES, and found the contents are almost the same (Supplementary Table 1 or Table R1). This evidence eliminates the leach of Ni.
In addition, in

2-The authors explained the loss of activity of Ru/TiO2 at higher temperatures by a loss of toluene
adsorption. Yet, H2 adsorption could also become limited at higher temperatures and may be the main origin of activity loss. This should be discussed.

Response to comments:
We thank this reviewer for the insightful comment. On Ni/Al2O3, Salmi and Smeds found the hydrogenation of toluene had the highest reaction rate at appropriate 170 o C and ascribed it to the escape of catalytically active hydrogen from the Ni-surface at the highest reaction temperatures. (Chemical Engineering Science, 1993, 48, 3813-3828). Meanwhile, Keane also observed the same trend of Ni/SiO2 in xylene hydrogenation that the TOF exhibited a maximum at 175 o C.
However, they ascribed it to a critical loss of the reactive aromatic species from the surface. (J. Catal. 1997, 166, 347-355) So in different systems, it's likely either H2 or toluene adsorption could be the reason that inhibiting the aromatics hydrogenation reaction at a higher temperature. Therefore, we agree with the reviewer's comment, and we have revised the explanation of "The significant activity loss of 2Ru/TiO2 with the temperature increasing from 120 o C to 200 o C can be ascribed to the continuously enhanced toluene desorption" to "The significant activity loss of 2Ru/TiO2 with the temperature increasing from 120 o C to 200 o C can be ascribed to the continuously enhanced toluene or hydrogen desorption". The two references have also been cited in the manuscript.

Reviewer #2
The paper by Wang et al. tackles a combination of very interesting and hot subjects, namely CO-tolerant Catalyst development for Crude Hydrogen Storage via the LOHC technology. This reviewer ranks the experimental catalysis, microscopic and spectroscopic work in this paper as ground-breaking, however, the conclusion drawn from the CO-tolerant hydrogenation studies as well as its reaction mechanism need further improvement. This is why, this reviewer ranks the paper as major revision' but would strongly support publication in Nature communications after addressing the open points in the results analysis section in an appropriate manner.

Response to comments:
We thank the reviewer for the very positive view of our work. Per the request of the reviewer, we have added additional experimental results and discussion in the following point-bypoint response and the revised manuscript.
Firstly, there are some confusions:  (2018) 104-114 107), so this reviewer suggests the authors make a clear discussion on this topic.

Response to comments:
We thank the reviewer for the comment. Based on the reference provided by the reviewer, we admit that under certain circumstance, the hydrogenation of aromatic substrate which generally requires metal ensemble sites can take place favorably at Pt single atom sites. In addition, it is also widely accepted that with less metallic character, metal single atoms tend to bind CO in a less intensive manner. Despite the theoretical potential of single atom catalysts (SACs) in driving the COtolerant toluene hydrogenation process, however, we believe the attenuated interaction between SACs and CO would inevitably, compromise the purification of the crude hydrogen feed. Alternatively, the parallel CO methanation along with toluene hydrogenation offers a more practical solution to alleviate the CO-poisoning effect of the metal to achieve simultaneous crude hydrogen storage and purification.
According to the above discussion, the second paragraph of the Introduction has been revised accordingly. We also cited those important references that the reviewer mentioned.
Secondly, about the reaction pathway and mechanism. The authors provided a beautiful study on microscopic and spectroscopic side and experimental catalysis side. But only a FT-IR is not efficient to bridging them up for reaction pathway and mechanism discussion.
1. The author should show the results of selectivity. Is there any by products such as methyl cyclohexene or -diene? The author should make a discussion on that.
Response to comments: We thank the reviewer for this suggestion. During the reaction process of 2Ru5Ni/TiO2, with toluene/CO/H2 as the inlet reactant, MCH was detected as the dominant toluene hydrogenation product (Supplementary Figure 3 or Figure R2). The only detectable byproduct is cyclohexene, with less than 0.2% selectivity, which can be regarded as trace amount. Figure R2. Typical GC analysis graph of CO-tolerant toluene hydrogenation using 2Ru5Ni/TiO2. Response to comments: We thank the reviewer for this insightful comment. To discuss the poison effect of CO on 2Ru5Ni/TiO2, we compare the catalytic hydrogenation performance of toluene in pure hydrogen and crude hydrogen (Figure 1a or Figure R3), while 0.1 % CO could induce the lower MCH yield especially for 2Ru/TiO2 and commercial catalysts ("However, along with the addition of 1000 ppm

In
CO…demonstrating the feasibility of crude hydrogen storage" in manuscript). Based on these results, we think the strongest poison species was CO in this reaction. For the pre-activation process, we also believe it is very important and can influence the catalytic performance greatly. So, during our DRIFTS experiments, we kept the pre-activation processes the same as real conditions. Although we think our DRIFTS results could elucidate the competing adsorption relationship of CO and toluene on the catalysts, we agree with the reviewer's opinion "directly bridging reactant adsorption with reaction activity is not efficient", and there are differences between the reactant adsorption and the reaction in true conditions.

To further clarify the adsorption issue, this reviewer suggests that a toluene and CO Temperatureprogrammed desorption on the three catalysts sample should be provide and discussed.
Response to comments: We appreciate this reviewer's comment. The CO-TPD on three catalysts samples has been added in the Supplementary Figure 27 (or Figure R5) and related discussion was added in Supplementary Note 4 and "The CO-TPD results show …the CO adsorption abilities of Ru on Ni" marked in blue in the manuscript. and m/z=28, Figure R6b), and the same trends of these two signals indicate that adsorbed toluene cracked to hydrogen, lower hydrocarbon and carbon deposition on the surface in the TPD conditions (Notably, in the real reaction conditions, the lower temperature and high partial pressure of H2 can inhibit this process). That is why we didn't observe the desorption of signal of m/z=92 and the date was not displayed in the SI. Figure R6. Toluene-TPD results of the reduced 2Ru/TiO2, 2Ru5Ni/TiO2 and 5Ni/TiO2 .

The authors provided a lot of characterization data on catalytic materials side. This reviewer suggests the authors to combine these data with catalysis to propose a reaction mechanism model. Such as combine the metal-support interaction, and metal (Ru)-metal (Ni) interactions to further explain their distinct binding properties. And provide more insight for the conclusion of "Ru and Ni served as the active sites for CO methanation and toluene hydrogenation respectively."
Response to comments: We thank for the reviewer for the insightful comment. The mechanism model (the following picture) has been added as

Is the interface of Ru-Ni matter for the reaction?
Response to comments: We appreciate this reviewer's comment, and it is an important question on the roles of Ru and Ni during reaction. From the related characterization results, we have drawn the conclusion that Ru has a stronger affinity to CO to help the CO activation and methanation, while Ni species, which are highly dispersed with the help of Ru, act as the main sites for toluene adsorption and hydrogenation in simulated hydrogen feed. As for the Ru-Ni interface, we think it is not the key point in this reaction. The reason is, as XANES results show (Figure 3c, d or Figures R8a, b)

Reviewer #3
The RuNi cluster on TiO2 wad design for stable activity for hydrogenation even with CO impurity. This paper can be accepted after considering followings.

Response to comments:
We thank the reviewer for the very positive view of our work. Per the request of the reviewer, we have added additional experimental results and discussion in the following point-bypoint response and the revised manuscript.

1) Ru and Ni can form solid-solution alloy or separated cluster?
Response to comments: We thank the comment from the reviewer, and this is a crucial question for the structure of our catalysts. Based on our experimental evidence, the Ru and Ni species are in separated phases with strong interactions rather than a solid-solution alloy due to the main reasons as followings.  To clarify this point, the previous description "NiRu cluster" is now changed to "Ni-Ru cluster" to highlight the separated phases.
2) The electronic state of Ru and Ni can be modified by the alloying or clustering?
Response to comments: We thank this reviewer for the insightful comment. According to the XANES result, the oxidation states of Ru are similar in 2Ru5Ni/TiO2 and 2Ru/TiO2 (Figure 3c or Figure R8a).
However, in the Ni K-edge of XANES (Figure 3d or Figure R8b), these two catalysts possess different oxidation states of Ni species in 2Ru5Ni/TiO2 and 5Ni/TiO2. As for 5Ni/TiO2, Ni is in a metallic state.
However, Ni shows oxidation evidence in 2Ru5Ni/TiO2 which should be attributed to the interaction with the adjacent Ru.
3) Why TiO2 is suitable support for the formation of this alloy cluster.

Response to comments:
We appreciate the reviewer's comment. The TiO2 support is important in stabilizing the RuO2 in 2Ru5Ni/TiO2 catalyst during calcination process. The d-spacings of rutile TiO2(110) (3.28Å) and RuO2(110) (3.23Å) are quite close, and this introduces an epitaxial relationship at the RuO2/TiO2 interface that should stabilize the RuO2 species and prevent the agglomeration.
In contrast, the interaction between Ni and TiO2 is relatively weak, and this is demonstrated by the agglomerated Ni species in 5Ni/TiO2 during the reduction process. However, with the addition of Ru, the migration of Ni is limited by the Ru obstacles that are stabilized on the TiO2 support, as shown in the schematic in Figure 2 (or Figure R10). Hence, TiO2 is a suitable support for the formation of highly dispersed Ru-and Ni-based clusters. Response to comments: We thank this reviewer for the suggestion. The reference recommended by the reviewer shows that TiO2 acts as efficient support for the synthesis of RuNi bimetallic alloy by simple impregnation followed by H2 reduction at 300 °C. In contrast, this RuNi bimetallic alloy cannot be effectively formed on other types of support (SiO2, Al2O3, ZrO2, MWCNT, Graphene, and MgO). As a result, the RuNi/TiO2 catalyst exhibits a remarkably high catalytic activity for the dehydrogenation from ammonia borane compared with those prepared catalysts with other supports.
Notably, in this reference, the content of Ru is higher than that of Ni for RuNi/TiO2 (Ru:Ni=1:0.3), and the formed alloy is a Ru-rich one (an alloy based on Ru-matrix). In contrast, the scenario in our work is a Ni-rich composition in 2Ru5Ni/TiO2, and this may introduce phase separation instead of alloying.
This result suggests that the Ru/Ni ratios should have an influence on the structure of RuNi species on TiO2.
Here, this reference has been cited in the revised manuscript, as the reviewer suggested.

Reviewer #4
The manuscript describes the development of bimetallic RuNi@TiO2 catalysts with the anti-COpoisoning ability for potential application in a LOHC-based   The total volume of the reactor is 10 mL.
b. 3 mL methyl-cyclohexane as the solvent, instead of cyclohexane.

Exemplar GC analysis of products from the rig is needed in the revised SI.
Response to comments: Thanks for the suggestion. Typical GC analysis graph has been added in Supplementary Figure 3 (or Figure R2). Response to comments: We thank the comment from the reviewer, and this is a crucial question on the performance of the catalysts. The activity decline during the 24 h continuous reaction did exist. We compare the weight loss of fresh and spent catalysts by TGA (Thermalgravimetric Analysis), and the result is shown in Figure R12. The weight loss of fresh 2Ru5Ni/TiO2 was 7 % and the spent catalyst was only about 2 %, which indicates the carbon deposition may not be the reason causing the activity loss during the reaction. However, through our STEM results, we found the growth of Ru and Ni species on the spent 2Ru5Ni/TiO2 (Supplementary Figure 10 or Figure R13). So the MCH yield drop was probably resulted from the metal particle agglomeration, and the detailed discussion has been added in the manuscript as "However, the slight MCH yield drop… of spent 2Ru5Ni/TiO2 catalyst" marked in blue.  6. Regarding the discussion for CO2 methanation, I think the authors oversell the performance. CO2 hydrogenation requires much higher temperatures than the current 100-220 o C, therefore, a simple discussion will be fine rather than being surprising about the phenomenon of non CO2 conversion in the current system.

Response to comments:
We appreciate this reviewer's suggestion. We have revised the statement in the manuscript regarding the catalytic performance of 2Ru5Ni/TiO2 in CO2 methanation properly.

Generic feature of the develop catalyst is needed, i.e. for hydrogenation of other LOHC pairs such as
DBT to PDBT and/or benzene to cyclohexane.

Response to comments:
We thank this reviewer for the insightful comment. We have extended the substrates besides toluene to benzene and p-xylene in a gas-solid fixed bed (Supplementary Figure 8 or Figure R14). In addition, the trials of benzene, biphenyl, phenyl and quinolone in batch reactor were displayed in Supplementary Table 2 (or Table R3). Apparently, the 2Ru5Ni/TiO2 catalyst shows the potential to realize the crude hydrogen storage over these organic substrates. And the sentences were added to the manuscript as "In addition, to prove the developed catalyst…which means the wide feasibility of the catalyst" marked in blue. For hydrogenation of DBT, the result was listed below ( Table   R4). As the products were hard to be distinguished by GC, so it was not shown in SI.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The authors have satisfactorily answered to my comments, both in the main article and in the supplementary material. Yet, I believe that it would strengthen further the paper if the authors mentioned that no Ni(CO)4 could be evidence during their experiments. This point is important on two aspects, first to strenghten the experimental data and second, as a safety matter, to stress that the evolution of such highly toxic compound (more toxic than HCN) should always be born in mind when using CO over nickel at relatively low temperatures.
Reviewer #2 (Remarks to the Author): The authors have made a great revision on this manuscript, the confusions were cleared up and more important experiments were added, which address the loosing point of the mechanism understanding. The reviewer agrees the publication it on nature communications.
Reviewer #3 (Remarks to the Author): Because moderate modifications were made, this can be accepted.
Reviewer #4 (Remarks to the Author): The authors have carefully considered all reviewers' comments and revised the manuscript thoroughly with many additional experiments to make the work more solid and interesting. Based on my previous overall positive comments on the work, I recommend publication of the current version of the manuscript.